Dump combustor with noncoherent flow

ABSTRACT

In a combustor in which a flow area for fuel and air increases suddenly at dump so as to generate vortices which mix the fuel and air for combustion downstream of the dump, the improvement in which the flow cross section upstream of the dump is noncircular, as triangular or rectangular, causing flow downstream of the dump to be noncoherent so that mixing by the vortices is aperiodic with the result that heat release on mixing is uniform in time and does not drive oscillations in the combustor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of reaction motors in which air issupplied by ram effect and in which a mixture of fuel and oxidizer isinjected into the reaction zone.

2. Description of the Prior Art

In a well known dump combustor, as used for example in a ramjet, fueland air mixed insufficiently for combustion flow through a duct having arelatively small cross section upstream of a "dump" at which the flowcross section increases substantially instantaneously generatingvortices which thoroughly mix the fuel and air for combustion downstreamof the dump. In prior art combustors of this type the cross sectionupstream of the dump, insofar as is known, is substantially circularresulting in coherent flow downstream of the dump so that such vorticesare uniform circumferentially of flow issuing from the smaller diametersection. These vortices, being uniform, form and break away at uniformintervals so that heat release upon mixing by the vortices is periodicwith resulting pressure oscillations in the combustor. If theseoscillations resonate with other accoustic oscillation natural to thecombustor, very high amplitude oscillations result with damage to thecombustor, an engine employing it, or the structure of a vehicleprovided with the engine. Such high amplitude oscillations may, in anyevent, perturb a guidance system of the vehicle or expel a desired shockfrom the inlet of such an engine causing an unstart thereof. As a resultthe advantages of such a prior art dump combustor, such as simplicity inconstruction and efficient combustion, are often not practicallyuseable.

SUMMARY OF THE INVENTION

A dump combustor in which, the flow area immediately upstream of thedump is substantially noncircular so that vortices induced by the dumpare aperiodic and flow downstream thereof is noncoherent so thatfuel/air mixing and the resulting combustion and heat release aresubstantially uniform in time and do not drive accoustic oscillations.

It is an object of the subject invention to provide a dump combustorhaving a configuration which does not result in periodic fluctuation inheat release in the combustor.

A further object is to provide a dump combustor which has the foregoingadvantage, is simple in construction, and provides efficient combustion.

BRIEF DESCRIPTION OF THE DRAWING

Other objects and advantages of the subject invention

be apparent from the following detailed description of the inventionwhen considered with the accompanying drawing in which:

FIG. 1 is a longitudinal section of a ramjet engine having a dumpcombustor;

FIG. 2 is a view taken from the position of line 2--2 of FIG. 1 showinga circular flow area used with such a dump combustor of the prior art;and

FIGS. 3A and 3B are views similar to FIG. 2 showing, respectively, atriangular flow area and a rectangular flow area utilized in such acombustor embodying the subject invention.

DETAILED DESCRIPTION OF THE PRIOR ART AND OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically depicts an exemplary ramjet engine having arepresentative dump combustor 10 The engine has an inlet 12 forcompressing supersonic airflow indicated by arrows 13, a substantialpart of the compression occurring in the inlet at a shock 14. Thecompressed air flows through a duct 16 to downstream portion 17, ofcombustor 10, portion 17 having a substantially larger cross sectionalflow area than duct 16. The transition or "dump" 18 between duct 16 andcombustor portion 17 is abrupt, so as to induce vortices, indicated inFIG. 1 by the numeral 19, in the shear layer between fluid in combustorportion 17 and fluid entering this portion from duct 16. Fuel, typicallyliquid, is injected as indicated by numeral 21 at a point somewhatupstream of dump 18. However, the air velocity and conditions ofinjection are such the air and fuel are not mixed effectively enough forcombustion in duct 16, although the region of duct 16 between the fuelinjection indicated by numeral 21 and dump 18 may be considered as anupstream portion 23 of combustor 10 contiguous with downstream portion17 thereof. However, immediately downstream of dump 18 the air and fuelare mixed sufficiently for combustion, the combustion occurring in theshear layer which contains vortices 19 during mixing thereby so thatcombustion is, in effect, retained in downstream portion 17. Aftercombustion the products thereof exhaust through a nozzle 25 as indicatedby arrows 26.

It should be noted that, in a dump combustor suited for practice of thesubject invention, there may be a plurality of inlet ducts correspondingto duct 16 entering an enlarged combustor portion corresponding toportion 17, that an inlet duct need not be coaxially related to such anenlarged portion, and that the direction of flow into such an enlargedportion need not be in the direction of other flow therein nor in thedirection of flow from a nozzle corresponding to nozzle 25.

In prior art dump combustors configured and functioning similarly tocombustor 10, the upstream portion 23, insofar as known to theapplicants, has, at dump 18, a substantially circular cross section, asindicated in FIG. 2 by numeral 30. As a result, vortices 31corresponding to vortices 19 in FIG. 2 are uniform circumferentially offluid entering enlarged portion 17 causing the oscillations andresulting previously described problems.

However, in the practice of the subject invention the upstream portion23 has, at dump 18, a substantially noncircular cross section such asthe rectangular cross section 35 shown in FIG. 3A or the triangularcross section 36 shown in FIG. 3B. As a result, the resulting vortices38, corresponding to vortices 19 in FIG. 1, are nonuniform in space andin time a in direction circumferentially of fluid entering enlargedportion 17 so that the above described mixing of fuel and air duringwhich combustion occurs does not have periodic fluctuations. The heatrelease in combustor portion 17 is thus substantially steady and doesnot drive accoustic oscillations in combustor 10 or other portions of anengine or vehicle having such a combustor. The noncircular cross sectionmay be provided by an orifice at dump 18, with duct 16 being circular,or duct 16 itself may be noncircular in cross section.

Obviously, other modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the following claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. In a dump combustor having an upstream portionthrough which effectively unmixed air and fuel flow, having a downstreamportion contiguous with said upstream portion and of substantiallylarger cross sectional flow area than the flow area of said upstreamportion, and having an abrupt transition between said flow areas toinduce vortices in a shear layer between fluid in said downstreamportion and fluid entering said portion from said upstream portion, saidvortices mixing the fuel and air immediately downstream of saidtransition with combustion of the fuel and air occurring during saidmixing, the improvement comprising said upstream portion having a crosssection which is substantially noncircular at said transition so thatsaid vortices occur nonuniformly in time in a directioncircumferentially of said fluid entering said downstream portion andsaid mixing does not have a periodic fluctuation, with the result thatheat release from such combustion is substantially steady and does notdrive accoustic oscillations in combustor.
 2. The improvement of claim 1wherein said noncircular cross section is substantially triangular. 3.The improvement of claim 1 wherein said noncircular cross section issubstantially rectangular.